One of the most difficult aspects of working with Drools is probably dealing with errors or unexpected behavior of a rule. The problem is even harder if we are talking about a knowledge base (KB) composed of multiple rules. Add interaction with external systems to the equation and see how the complexity grows even further.

The declarative nature of Drools also presents some unique challenges when testing our code. Unlike classes and functions, rules can't be directly invoked. If we want to test a single rule, we need to recreate the necessary state of the session required by the left-hand side of this rule. The problem is that the same state could also trigger other rules in the same knowledge base. This is the reason why, usually, when testing a knowledge base, we test complete scenarios that may involve multiple rules.

In Drools, there is no magic bullet that allows us to easily test and debug our rules; instead, what we have is a set of good...